In a train vehicle control and occupancy detection system such as disclosed in U.S. Pat. No. Re. 27,472 of G. M. Thorne-Booth, audio frequency signals are operative with predetermined track signal block circuits for the purpose of sensing the occupancy by a train vehicle of a signal block circuit and are operative for fail-safe speed control of that train vehicle. The track rails are divided into signal block circuits of known length by low impedance connections placed across the track rails at the ends of each signal block. A signal transmitter is provided at one end of each signal block to supply a desired speed control signal to that signal block and a first signal receiver is provided at the opposite end of the signal block to determine vehicle occupancy and a second signal receiver is carried by the train vehicle to determine the operating speed of the vehicle in that signal block. For this purpose comma free binary coded vehicle control signals and vehicle speed command information is transmitted on multiple message frequencies, such as 5 kilohertz and 10 kilohertz which represent binary message units 1 and 0. Digital frequency modulation, frequency shift key modulation or phase shift key modulation can be employed to carry binary coded speed command information to the train vehicle. Before the information transmitted on the message frequencies can be utilized it must be decoded according to the particular modulation used by the control system. This decoding must be performed in a manner such that extraneous signals which are received will not cause the vehicle to operate in an unsafe manner.
There is disclosed a fail-safe amplifier apparatus having a predetermined gain in U.S. Pat. No. 3,838,353 of T. C. Matty, wherein the feedback impedance included a four terminal capacitor to compensate for the failure mode of the amplifier in which the open circuit condition of the feedback element occurs or a short circuit condition occurs.